I would be rather baffled. While it is more than possible to have all four of them unique, it is both rather selfish of the series to take up four styles of swordsmanship, and personally I think it would be a lame representation to have four characters of what is essentially two core designs. I am fine with Marth and Chrom (or Ike or Lucina) because the series doesn't have much in the way of variety in lead characters, but if we get a third or even a fourth, I would want something that shows off as much of the series as possible. Marth, Chrom, Robin and Anna perhaps? Honestly in the grand scheme of Nintendo, unless we get over 50 characters (and I sincerely hope not), three is more than enough.
The "merit" is non-existant.
I think that is unreasonable.
If not for it, Ike could have been Sigurd. Ike could have been Alm, or Hector, as they could all be stretched to fill that fighting style. After all; Ike was "fitted" to a character type they didn't have. Sure RD Ike could stastically be argued as a sluggish character, as far as presentation is considered in all of his appearances and his stats in PoR, it betrays him entirely. In way of presentation he is very extremely nimble, but he is also very strong and as such can be tailored to a stronger swordsman. Ike could have been Ganondorf and if they wanted to maintain a quota of two Fire Emblem characters, we could have had someone completely different.
No one is saying recency is the song that brings the cows home, but the developers certainly value it. They also value gameplay potential, aesthetic distinctiveness, contrast to other characters, relation to other characters, etc. Link has appeared in Smash Bros as the most recent Link three times. Now before you retort by saying that is because Link is always Link and fulfills the quota for what the fans want, I am just saying they took recency into consideration. If something like popularity or notoriety was the whole story, we would probably see a repeat of OoT Link or the emergence of LttP Link. Merit is a cumulative set of values. Of course they are subjective, and the director has to decide what is most important, usually in guidance with the creator of said character and sometimes popular opinion.
If Ike never showed up and that style of character never made an appearance, Chrom could have been "Ike". He can more than easily fulfill the exact same gameplay to a tee. But Ike did, and soon enough made his way into people's hearts. Appearing in Smash Bros, like recency, is a merit. I doubt anyone would be asking for Roy as much as they have if he didn't appear in the series, either. In fact if he wasn't the most fresh on the minds of Nintendo, it would have been simple enough for them to use a myriad of other characters. Despite inspirations from Marth and Ike, Chrom can just as easily be the next flavour of the month for people who are completely ignorant of the series prior.